Hereinafter, a “Q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit circuit. Any reference to the term “qubit” is a reference to a superconducting qubit circuitry that employs a Josephson junction, unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and |1>. These two states may be two energy states of atoms, for example, the ground (|g>) and first excited state (|e>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states are allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the |0> and |1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
A quantum processor chip (QPC) can contain one or more qubits. A QPC can have one or more lines for microwave signal input or output. A common non-limiting embodiment of a microwave line is a coaxial cable carrying electromagnetic signal in the microwave frequency range.
Because presently available QPCs operate at ultra-low cryogenic temperatures, the lines, the readout circuits, and other peripheral components used in a quantum computing environment pass through one or more dilution refrigerator stage (compactly referred to herein as a “stage”). A stage operates to decrease the thermal state, or temperature, of lines and components entering at a high temperature side of the stage to the stage temperature—a temperature maintained at the stage. Thus, a series of stages progressively reduce the temperature of a line from room temperature (e.g., approximately 300 Kelvin (K)) to the cryogenic temperature at which the qubit operates, e.g., about 0.01 K.
A line from the final (lowest temperature) stage couples to the QPC. A signal from the qubit is conversely carried out on a line whose temperature progressively increases as the line passes through the series of stages in the direction away from the QPC. At each stage, including the final stage, the line has to connect to a semiconductor or superconductor circuit.
A microstrip is a planar conductive structure in which a conducting material is formed in the shape of a strip on one side of a dielectric substrate, with a ground plane on the opposite side of the substrate. A ground plane is a structure—often a conductive metallic structure—at a ground potential. The strip forms a conductor of the microstrip (hereinafter compactly and interchangeably referred to as a “conductor”, “microstrip conductor”, “MC” and their plural forms). Although commonly the conductor is formed in the forms of a substantially rectangular prism—having a substantially rectangular cross-section and a length—the illustrative embodiments contemplate other forms, such as cylindrical wires, also being formed and used as the conductor in a microstrip of an embodiment described herein.
Presently, a microstrip is used to couple a signal transmission line to a circuit. The illustrative embodiments recognize that the presently, microstrips and the methods of forming them is not suitable for quantum applications for a variety of reasons. For example, most microstrips that are fabricated in common dielectric substrates materials are usable only below 1 Gigahertz (GHz) and are not usable at cryogenic temperatures, particularly at temperatures below 4 K. Qubits operate at above 1 GHz and at temperatures significantly below 4 K. The microstrips that are fabricated using superconducting materials can operate below 4 K and above 1 GHz but are poor thermal conductors and are not suitable for soldered connections to lines.
The illustrative embodiments recognize that for a microstrip to be usable in a quantum computing environment, the microstrip should thermalize well within the stage. Thermalization of one structure to another structure is the process of constructing and coupling the two structures in such a way that the coupling achieves at least a threshold level of thermal conductivity between the two structures. Good thermalization, i.e., thermalization where the thermal conductivity between the thermally coupled structure exceeds the threshold level of required thermal conductivity. For example, a thermal conductivity of greater than a 1 Watt/(centimeter*K) at 4 Kelvin, is an acceptable threshold level TH of good thermal conductivity according to the illustrative embodiments.
The illustrative embodiments recognize that a manner of coupling a microwave line to a circuit in a stage or to a qubit should exhibit good thermalization, good electrical conductivity (e.g., exhibit a threshold (TC) Residual Resistance Ratio (RRR) of at least 100), and provide this electrical and thermal performance at cryogenic temperatures down to a millikelvin and lower, e.g., to 0.000001 K. Furthermore, the manner of coupling should be reliably usable in soldered connections.
The illustrative embodiments recognize that presently formed microstrips, when used for microwave applications cause a significant crosstalk between adjacent microstrip conductors. Because the quantum applications are dealing with levels of energy as small as a single photon, microwave interference from crosstalk and other noise must meet far more stringent requirements than in non-quantum applications. For example, for microstrips to be usable in quantum applications, the crosstalk between MCs should be less than −50 decibels (dB).